The present invention relates to intermediate layers of inorganic materials in electroluminescent arrangements or devices respectively, containing finely divided inorganic particles (nano-particles) dispersed in a polymer binder.
Current-operated, light-emitting arrangements are of great importance in our opto-electronic age. The development of optical display elements aims towards arrangements which are flat, can be operated with a large area, are highly efficient and are low in production costs. There are a number of different possibilities of producing electrically operated light-emitting arrangements. However, only two classes of system are known which combine the requirements of large area, flatness and cheap production.
1) Doped ZnS Particles in a Binder Matrix
The ZnS particles are incorporated into a thin layer and luminesce upon the application of a high-frequency electric field. The luminance of the ZnS systems is low. A further disadvantage consists in the high operating voltage required at a high frequency.
2) Electroluminescent Polymers
Polymer LEDs based on poly(phenylene)vinyls were described for the first time by Friend et al (U.S. Pat. No. 5,247,190). In the simplest construction the polymer is arranged between a transparent anode (e.g. ITO) and a vapour-deposited cathode (e.g. Ca, Al). The arrangement emits light when a voltage is applied.
The most general example of a layer construction in an organic LED is described in EP A-0 637 899 wherein additional layers, which exert a positive influence upon the function of the light-emitting arrangements, are also described. The properties of LEDs constructed from organic components can be distinctly improved by the introduction of additional organic layers (Tang et al. Appl. Phys. Lett. 51 (1987) 913). For this purpose one or two charge-injecting layers are introduced between the electroluminescent layer and the two electrodes. These charge-injecting layers can consist of vapour deposited monomers (Adachi et al. Appl. Phys. Lett. 57 (1990) 531), polymers (U.S. Pat. No. 5,231,329) or monomers dispersed in polymers (Brown et al. Appl. Phys. Lett. 61 (1992) 2739). These are for example oxadiazoles or tertiary amines. Polymers with oxadiazole structures were also described. The molecules are always organic.